Paleomagnetic Determination of Vertical-Axis Block Rotation and Magnetostratigraphy in the Mecca Hills and Coachella Valley, California

Our ongoing work on the paleomagnetism and magnetostratigraphy in the Coachella Valley has provided an improved understanding of the timing and spatial variations of sediment accumulation and deformation during evolution of plate-boundary fault zones. Here, we report updated results from the Palm Spring Formation of the Mecca Hills, and new paleomagnetic data from Pleistocene conglomeratic sandstone in Desert Hot Springs and the Plio-Pleistocene San Timoteo Formation from Live Oak Canyon. From the Mecca Hills, new data were obtained-112 samples from 29 sites. The majority of the paleomagnetic results yielded well-defined components of magnetization, which allowed us to identify seven well defined polarity zones within the Ocotillo and upper Palm Spring Formation. These polarity zones are correlated with the geomagnetic polarity timescale using the Bishop Ash near the top of the section as a tie point. This correlation places the Brunhes-Matuyama boundary at or near the base of the Ocotillo Formation, with the Jaramillo, Olduvai, and Reunion normal polarity sub-chrons occurring in the upper Palm Spring Formation. This indicates that the upper Palm Spring Formation in the Mecca Hills was deposited between ca. 2.3 and 0.9 Ma. Tilt corrected mean directions from the Mecca Hills section are D = 343, I = 53, α95 =11.3 N = 5 for normal sites, and D = 175, I = -50, α95 = 4.9 N = 24 for reverse sites. Normal and reverse sites combined as a single mode have a mean direction of D = 353, I = 51 α95 = 4.4. These mean directions indicated modest (7 degrees) of CCW rotation with no significant variation in amount or sense of rotation observed within the section. Specimens from 19 sites (53 samples) of the Pleistocene conglomeritic sandstone from Desert Hot Springs have very well-defined paleomagnetic components. Six of the sites have normal polarity, 13 sites have reverse polarity. Sites with normal polarity have a mean direction of D = 358, I = 45, α95 = 13 and reverse sites have a mean of D = 182, I = -50, α95 = 6.6.The combined mean direction (in tilt-corrected coordinates) is D = 0.7, I = 49, α95 = 5.6, supporting 3.1° ± 2.3° of CW rotation at this location since ~1 to 1.5 Ma. Specimens from 8 sites (35 samples) of the upper-most San Timoteo Formation from Live Oak Canyon also have well-defined paleomagnetic components for 6 sites. All of the results have normal polarity, and one site has a direction that is \u3e40 degree from the other sites. The mean of the remaining 5 sites is D = 11, I = 49, k=51, a95=11. The new and updated paleomagnetic results from the Coachella Valley collectively indicate that this area has experienced modest (in most cases less than 10 degrees) CW or CCW rotation during the past 1-2 Ma. The lack of variation in amount or sense of rotation as a function of age suggest that rotation has been relatively recent (during the past ~ 1 Ma)